The dynamic behavior of magnetic colloids in suspension

Tina Autenrieth,Gerhard Grübel,Aymeric Robert,Joachim Wagner

doi:10.1107/s0021889807009016

Abstract

The dynamic behavior of magnetic colloidal particles in suspension is investigated. The particles of the core–shell colloid consist of a cobalt ferrite core embedded in a silica shell and are stabilized by surface charges. As the suspension is strongly opaque to visible light, it can not be probed by dynamic light scattering techniques as a result of strong multiple scattering as well as absorption effects. Therefore, the static and dynamic behavior is probed with small-angle X-ray scattering and X-ray photon correlation spectroscopy (XPCS), respectively. Using XPCS, we are able to study the diffusion coefficient of an opaque colloidal system as a function of the scattering vector. In this paper, we report on the behavior of the colloidal system in the absence of an external magnetic field, meaning that the magnetic moments of the particles are oriented randomly. We find no evidence for magnetic interactions in the static data, while the dynamic XPCS data deviate very significantly from the predictions of model calculations.

Highlights

The synthesis of highly defined colloidal particles with the ability to self-organize in suspension has provided us with interesting model systems for soft condensed matter
The static and dynamic behavior is probed with small-angle X-ray scattering and X-ray photon correlation spectroscopy (XPCS), respectively
We report on the behavior of the colloidal system in the absence of an external magnetic field, meaning that the magnetic moments of the particles are oriented randomly

Summary

Introduction

The synthesis of highly defined colloidal particles with the ability to self-organize in suspension has provided us with interesting model systems for soft condensed matter. In contrast to hard-sphere colloids, where self-assembly occurs at relatively high volume fractions ( > 0:49), an ordering of charge-stabilized particles can be observed s250 Tina Autenrieth et al Magnetic colloids at volume fractions even below 0.01 (Hartl et al, 1999). The observed deviation of the dynamic data from theory (pairwise-additive approximation) points to a deficiency of the model for charge-stabilized suspensions at non-vanishing concentrations rather than to an effect of magnetic interactions. G1ðQ; Þ is the field autocorrelation function [ called the intermediate scattering function f ðQ; tÞ] that can be expressed for a diffusion process of non-interacting (Brownian) spherical particles by a single exponential decay, g1ðQ; Þ 1⁄4 exp1⁄2ÀÀðQÞ; ð5Þ with the relaxation constant ÀðQÞ 1⁄4 Q2DðQÞ. For hard-sphere systems, the expansion [a many-body approach proposed by Beenakker & Mazur (1984)] is an appropriate theory to describe hydrodynamics

Instrumentation

Synthesis of the core–shell colloids

Results and discussion

Conclusion